1. Field of the Invention
The present invention relates to a photomask used to produce high-density integrated circuits such as LSIs and VLSIs and to a photomask blank used to produce the photomask, and, particularly, to a halftone phase shift photomask enabling the formation of a finely dimensional projected image and to a halftone phase shift photomask used to produce this phase shift photomask.
2. Prior Art
Semiconductor integrated circuits such as ICs, LSIs, VLSIs and the like are produced by repeated lithographic steps using a photomask. In order to attain, particularly, fine dimension, investigations on the use of a phase shift photomask such as those shown in, for example, EP 0 090 924A2 (JP-A-58-173744 and JP-B-62-59296) are being made. There have been proposals concerning phase shift photomasks having various structures.
Among these photomasks, halftone phase shift photomasks as shown in, for example, JP-A-4-136854 and U.S. Pat. No. 4,890,309 attract attention from the viewpoint of early practical use.
There have been a few proposals concerning an improvement in yield by a reduction in the number of production steps and structures and materials which can reduce costs as described in JP-A-5-2259 and JP-A-5-127361.
As will be explained in detail hereinbelow, this halftone phase shift photomask comprises a transparent substrate, a halftone phase shift film and as required, a light shading film.
In the meantime, as thin film materials for photomask patterns, molybdenum silicide as described in U.S. Pat. No. 4,985,319 (EP 0 190867, JP No. 1750121), U.S. Pat. No. 4,783,371 (JP No. 1781105) is generally known. Because its processing characteristics are superior, energetic trials have been made to apply molybdenum silicide to halftone phase shift films by oxidizing or nitriding molybdenum silicide as shown in, for example, JP-A-6-83027 and U.S. Pat. No. 5,474,864 (JP-A-7-140635).
However, molybdenum silicide is usually treated by dry etching using fluorine type etching gas such as CF4, CHF3, SF6, C2F6, NF3, CF4+H2 and CBrF3. At this time, there is the problem that a transparent substrate using synthetic quartz as a substrate material is also etched and therefore highly precise dry etching cannot be attained.
Generally, as to the production of a halftone phase shift photomask, it is essential to control the phase angle with high accuracy. However, as aforementioned, the quartz substrate is also etched when etching the halftone phase shift film, with the result that an error in phase difference is produced corresponding to the amount to be etched.
Also, the etching of a halftone phase shift film has an important role in a pattern size control. It is therefore desired to set conditions so as to obtain better uniformity and reproducibility of a pattern dimension as much as possible. This adds a new parameter, namely etching selection ratio of the phase shift film to quartz, giving rise to the problem that a margin for condition setting is narrowed.
This is a problem which arises because an optimum etching process for dimensional control does not always accord to an optimum etching process in which the above phase difference control is regarded as important.
Specifically, this is a problem that although a molybdenum silicide type halftone phase shift film material itself exhibits superior processing characteristics and chemical stability after processed, highly precise patterning is made difficult taking also the highly precise control of phase difference into account.